1. Field of the Invention
The present invention relates generally to a hydraulic anti-skid device for use in a hydraulic braking system of a vehicle.
2. Description of the Prior Art
As is well known in the art, a hydraulic anti-skid device of this type is usually provided in a passageway supplying a hydraulic fluid pressure from a master cylinder to a brake unit such as a wheel cylinder and controls the supply of the hydraulic fluid pressure to the brake unit by means such as, for example, an electronic signal and a hydraulic fluid pressure fed by a pump and controlled by a regulator valve.
In the event of braking of a vehicle being effected by depressing a brake pedal thereof, when it is unnecessary to reduce the hydraulic fluid pressure fed to the brake unit as when the deceleration of the vehicle is not excessively great, the anti-skid device allows the hydraulic fluid pressure from the master cylinder to pass to the brake unit. Conversely, when it is necessary to reduce the hydraulic fluid pressure fed to the brake unit as when the deceleration of the vehicle is excessively great, a predetermined electronic signal is generated and the anti-skid device prevents the hydraulic fluid pressure from the master cylinder from being fed to the brake units and expands the fluid retained in the wheel cylinders to reduce the braking force exerted thereby.
Also, when the hydraulic fluid pressure from the pump is not fed because of a malfunction, the anti-skid device provides communication between the master cylinder and the brake unit irrespective of the presence and absence of the electronic signal. Such a hydraulic anti-skid device is described in detail below in connection with FIG. 1 of the accompanying drawings.
As shown in FIG. 1, a hydraulic anti-skid device 10 includes a control block 12 formed with ports P.sub.1, P.sub.2, P.sub.3, P.sub.4 and P.sub.5. The port P.sub.1 is connected to a master cylinder A, the port P.sub.2 is connected to a brake units B (only one shown), the port P.sub.3 is connected to a pump C, the port P.sub.4 is connected to a reservoir tank D, and the port P.sub.5 is connected to a power steering system (not shown) which is not directly related to the anti-skid device.
The port P.sub.1 is communicable with the port P.sub.2 on the one hand through passages 14, 16 and 18 and on the other hand through a passage 20 and the passage 18. The passage 20 is arranged in parallel with the passages 14 and 16. An expansion valve 22 is disposed so as to control communication between the passages 14 and 16 and a bypass valve 24 is disposed so as to control communication between the passage 18 and both the passage 16 and 20. The expansion valve 22 has a portion exposed to a chamber 26, while the bypass valve 24 has a portion exposed to a chamber 28.
The port P.sub.3 communicates on the one hand with the port P.sub.5 through a regulator valve 30 and a passage 32 and on the other hand with the chamber 26 through the regulator valve 30, a passage 34 and a passage 36. The passage 36 has formed therein an orifice 38. An electromagnetically operated armature valve 40 is provided for opening and closing the passage 36. The passage 34 is branched off to a passage 42 which communicates with the chamber 28. The port P.sub.4 is communicable with the chamber 26 through a passage 43.
The regulator valve 30 regulates the hydraulic fluid pressure, fed from the pump C into the passages 32 and 34, in accordance with the hydraulic fluid pressure fed from the master cylinder A into a passage 44 which communicates with the passage 14.
The bypass valve 24 is moved into a left-hand position shown in the drawing by the fluid pressure fed into the chamber 28 through the passages 34 and 42 so that a differential valve 45 integral with the bypass valve 24 obstructs communication between the passages 18 and 20 and provides communication between the passages 16 and 18 through a chamber 46. When the fluid pressure is not fed in the chamber 28, the bypass valve 24 is pushed into a right-hand position in the drawing so that it provides communication between the passages 18 and 20 through the chamber 46 and obstructs communication between the passages 16 and 18. The bypass and differential valves 24 and 45 functions as a safety valve for ensuring a braking operation when a trouble take places in the pump C and/or the passage 34, 36 or 42.
The armature valve 40 includes an armature 48 provided with an armature ball 50 which normally closes the passage 43. A spring 52 urges the armature 48 into a position shown in the drawing in which the armature ball 50 closes the passages 43. A solenoid coil 54, when is fed with an electric current and is energized, attracts the armature 48 and the armature ball 50 to move them into a position in which the armature ball 59 opens the passage 43. When the armature ball 50 closes the passage 43, the fluid pressure is fed from the passage 34 into the chamber 26 through the passage 36, while when the armature ball 50 is attracted together with the armature 48 by the solenoid coil 54, the armature 48 obstructs communication between the chamber 26 and the passage 36. The solenoid coil 54 is electrically connected to an electronic control circuit (not shown) to receive therefrom an output signal for energizing the solenoid coil 54 when it is necessary to reduce the fluid pressure applied to the brake units B as when the deceleration of the vehicle has been excessively increased.
The expansion valve 22 is, when the fluid pressure from the passage 34 is fed in the chamber 26, pushed by the fluid pressure into a left-hand position shown in the drawing in which a check valve 55 of the expansion valve 22 is opened to provide communication between the passages 14 and 16 through a chamber 60. The check valve 55 comprises a valve 56 fixedly secured to the expansion valve 22 and a valve 58 engageable with and disengageable from the expansion valve 22. When the expansion valve 22 is in the left-hand position, the valves 56 and 58 are disengaged respectively from the valve 58 and a valve seat formed in the chamber 60 or the passage 16. The expansion valve 22 is, when the fluid pressure is absent in the chamber 26, moved in a right-hand position in the drawing in which the check valve 55 is closed to obstruct communication between the passages 14 and 16. At this time, the valves 56 and 58 are engaged respectively against the valve 58 and the valve seat.
The anti-skid device 10 thus described is operated as follows:
1. Normal operating period (when a trouble or malfunction does not occur anywhere in the anti-skid device 10 and the pump (C).
(1) When the brake pedal 62 of the vehicle is not depressed so that the master cylinder A does not generate any fluid pressure (viz.fluid pressure is not fed into the port P.sub.1), the expansion and bypass valves 22 and 24 are moved respectively into the left-hand positions shown in FIG. 1 with the control pressure from the pump C acting on the valves 22 and 24 on their right sides. As a result, the expansion valve 22 provides communication between the passages 14 and 16 and the bypass valve 24 provides communication between the passages 16 and 18 and obstructs communication between the passages 18 and 20. On the other hand, the regulator valve 30 is moved into a left-hand position in the drawing with the fluid pressure from the pump C acting on the valve 30 on its right side.
(2) When the brake pedal 62 is depressed so that the fluid pressure is fed from the master cylinder A into the chambers 60 and 46 and the passage 44 and is increased above a predetermined value to increase the pressure in the chambers 46 and 60, the fluid pressures in the chambers 60 and 46 and the passage 44 act respectively on the expansion, bypass and regulator valves 22, 24 and 30 on their left sides in the drawing to increase the forces urging the valves 22, 24 and 30 rightwards. In this instance, the regulator valve 30 is moved some amount rightwards by the fluid pressure acting on the valve 30 from the passage 44 so that the fluid pressure in the passage 34 is increased by an orifice effect. In this state, the regulator valve 30 is moved to a position where equilibrium is established. In the case of the bypass valve 24, although the fluid pressures act respectively on the right and left sides of the valve 24, since the force pushing the valve 24 leftwards is greater than the force pushing the valve 24 rightwards, the bypass valve 24 is held in the left-hand position so that it provides communication between the passages 16 and 18 and obstructs communication beween the passages 18 and 20.
In the case of the expansion valve 22, since the solenoid coil 54 is not energized when the signal for reducing the braking pressure is not generated as when the deceleration of the vehicle is not excessively great, the armature 48 and the ball 50 are held in an upper position shown in FIG. 1 in which the ball 50 closes the passage 43. As a result, the fluid pressure is fed from the passage 34 into the chamber 26 to push the expansion valve 22 and the check valves 56 and 58 leftwards. Since the force pushing the valves 22, 56 and 58 leftwards is greater than the force pushing the expansion valve 22 rightwards due to a relatively large area of the valve 22 which is acted on by the fluid pressure in the chamber 26, the expansion valve 22 is held in the left-hand position shown in the drawing.
Accordingly, the fluid pressure from the master cylinder A is conducted into the wheel cylinder B by way of the passages 14, 16 and 18 and the chambers 60 and 46.
(3) The event of the signal for reducing the braking pressure being generated.
When the braking pressure is increased and the deceleration of the vehicle exceeds a predetermined value, the solenoid coil 54 is fed with an electric current and is energized. This electric current is generated in and is controlled by a computer circuit in a module (not shown). When the solenoid coil 54 is energized, the armature 48 is moved by a magnetic force together with the armature ball 50 downwards in the drawing to open the passage 43. As a result, the fluid pressure acting on the right side of the expansion valve 22 is returned into the reservoir D through the port P.sub.4 to reduce to zero and the expansion valve 22 is pushed rightwards by the braking pressure acting on the left side of the valve 22. At this time, the valves 56 and 58 of the check valve 55 are closed to obstruct communication between the passages 14 and 16. Accordingly, the hydraulic fluid confined in the piping for the wheel cylinders B expands by a volume formed by the rightward movement of the expansion valve 22 to reduce the fluid pressure in the piping. Accordingly, the braking force is removed or reduced so that skid is prevented. The bypass and regulator valves 24 and 30 are both held in the same positions as those in the case of (2) mentioned above.
2. The event of a trouble occurring in a pressurized hydraulic fluid circuit including the pump C, the piping such as the passages 32, 34, 36 and 42, and so on.
In this instance, since the fluid pressure in the circuit is reduced to zero, the expansion, bypass and regulator valves 22, 24 and 30 are all moved by the braking pressure into their right-hand positions, respectively, in the drawing. As a result, the check valve 55 obstructs communication between the passages 14 and 16 and the differential valve 45 obstructs communication between the passages 16 and 18 and provides communication between the passages 20 and 18 through the chamber 46 to directly connect the master cylinder A and the brake units B. Accordingly, in spite of the malfunction of the pressurized hydraulic fluid circuit, the braking operation is made possible to assure the safety of the vehicle. However, in this instance, even if the signal for reducing the braking pressure is generated, a skid control operation is not provided.
In the conventional anti-skid device 10 thus described, since, when the pump C is not operated, the hydraulic fluid pressure is absent in each of the chambers 26 and 28, the expansion and bypass valves 22 and 24 are both moved rightwards by the hydraulic fluid pressure from the master cylinder acting on the left side thereof. As a result, the check valve 55 is closed to obstruct communication between the passages 14 and 16 and the differential valve 45 obstructs communication between the passages 16 and 18 as mentioned hereinbefore. Thus, since the passage 16 is completely isolated from the outside of same, it is impossible to remove air in the passage 16 in this state. Accordingly, it has been necessary to perform a work of removing air in a condition in which the engine is made running to operate the pump C and to move the expansion and bypass valves 22 and 24 by the hydraulic fluid pressure from the pump C into their left-hand positions shown in the drawing. This is undesirable in the safety of the operator performing the air removing work.